1. Field of the Invention
This invention relates to a simple, light, small, long-life, high color rendering property white color light emitting device which is suitable for lighting, displaying and liquid crystal backlighting.
This application claims the priority of Japanese Patent Applications No.2002-139865 filed on May 15, 2002, No.2002-153447 filed on May 28, 2002 and No.2003-42030 filed on Feb. 20, 2003, which are incorporated herein by reference.
Plenty of light emitting diodes (LEDs) and laser diodes (LDs) have been widely produced and sold on the market as small, long-life, inexpensive light emitting devices. High luminescence light emitting diodes (LEDs) have been already obtained for red, yellow, green and blue. Red light emitting diodes (LEDs) are LEDs having AlGaAs active layers or GaAsP active layers. The active layer means a thin layer which produces and emits light. Energy of emitted light is equal to the bandgap of an active layer. Color of emitted light depends upon the bandgap of the active layer. Yellow and green light can be produced by GaP-LEDs having GaP active layers. Orange/yellow light can be yielded by LEDs having AlGaInP active layers.
Production of blue light which requires a wide bandgap material had been one of the difficult problems. SiC (silicon carbide) type LEDs, ZnSe (zinc selenide) type LEDs and GaN (gallium nitride) type LEDs which had wide bandgap active layers had competed with each other for accomplishing practical, high luminescent, long lifetime blue light LEDs for a while. High-luminescence and long lifetime had allowed the GaN type LEDs to win a victory in the blue light LED race. Gallium nitride type light emitting diodes (GaN-LEDs) have an indium gallium nitride (InGaN) active layer. InGaN is a mixture crystal of InN (indium nitride) and GaN (gallium nitride). A mixture rate x, which means the ratio of components, is omitted here. The GaN type LEDs are denoted by InGaN-LEDs or GaN-LEDs hereafter. The InGaN-LEDs are made upon sapphire substrates (Al2O3). All the light emitting diodes (LEDs) or laser diodes (LDs) which produce light by electron bandgap transitions emit light of a single color whose energy is equal to the bandgap of the active layer. Monochromatic emission is one of the excellent features of semiconductor light emitting devices (LEDs & LDs) which make use of the bandgap transitions. Semiconductor light emitting devices (LEDs & LDs) are inherently monochromatic light sources. Monochromacity, however, forbids semiconductor devices from generating light including a plurality of colors. No single semiconductor light emitting device can yield complex color light.
2. Description of Related Art
Monochromatic light sources are useless as illuminating light source. Monochromatic light sources are unsuitable for a liquid crystal backlight for display. Illumination requires white color light sources, in particular, white light sources of high color rendering properties. Liquid crystal backlight, which should generate full colors, also requires white color light sources of high color rendering properties. At present, incandescent light bulbs or fluorescence tubes have been still used as illuminating light sources prevalently. Incandescent light bulbs are favorable for illuminating sources due to high color rendering properties. But incandescent light bulbs have drawbacks of a short lifetime, low efficiency and big volume. Fluorescence tubes have weak points of a short lifetime, heavy weight and large bulk.
White color light sources of a small size, long lifetime, high efficiency and low-cost are desired for illuminating, liquid crystal backlighting and displaying light sources. Nothing else than semiconductor devices can satisfy difficult requirements of a light weight, small size, long lifetime and high efficiency.
At present, blue light LEDs, green light LEDs and red light LEDs are sold on the market. Three elementary color LEDs are available. An assembly of blue, green and red color LEDs mounted on a common panel will be a compound white color light source. The three elementary color mixing LEDs have already been proposed and partly put into practice. However, such a compound white color LED has drawbacks. Since the three types of LEDs emit different colors (G,R,B), the LEDs should be densely populated on the common panel for making white. If the different color LEDs were sparsely dispersed, human eyes discern three kind individual colors instead of white.
The three types of LEDs have different properties of currents, voltages and emission efficiencies, which requires three different electric power sources. Luminosity of three kind LEDs should be balanced for making desired white. In addition, an array of many sets of three kind LEDs at high density enhances cost.
No high cost light sources pervades. Expensive white light sources are useless. Low cost and small sized white color light sources should be produced as semiconductor devices. Instead of assembling three different LEDs, a simpler structure containing a single LED is required for making low cost devices. Prior art of single LED devices is described. One is a complex LED which includes an on-sapphire blue light InGaN-LED and a YAG(yttrium aluminum garnet) fluorescence material enclosing the InGaN-LED. The InGaN-LED makes blue light. The YAG material fluoresces yellow light by being irradiated by the InGaN-LED blue light. Blue light and yellow light synthesize white light. This known device is simply called a GaN-type white color light source device (A).
The other is a blue light ZnSe-LED having an n-type ZnSe substrate doped with a special impurity and an ZnCdSe active layer grown above the substrate. The impurity-doped ZnSe substrate acts as a kind of fluorescence material which induces an SA (self-associated) emission by the blue light of the ZnSe-active layer (ZnCdSe). Yellow light omitted from the AnSe substrate and blue light induced from the ZnCdSe layer synthesize white light. This known device is called a ZnSe-type white color light source device (B). (A) and (B) are described in detail.
(A-type) GaN-Type White Color Light Source Device (YAG+InGaN-LED; FIG. 1)
GaN-Type Device (A) was Proposed by,
{circle around (1)} “White Color Light Emitting Device”, edited by the committee of Manual of photoactive materials, optoelectronics corporation, p457, June 1997
FIG. 1 shows the structure of the proposed the white color device (A).
A Γ-shaped lead 2 includes a horizontal top part which has a cavity 3. An InGaN-LED 4 is epi-up fixed at an bottom of the cavity 3. A resin 5 including a Ce doped YAG fluorescent material is supplied to the cavity 3. The YAG fluorescent material has a role of absorbing blue light and emitting yellow light of lower energy with a broad spectrum. The lower energy light produced by electrons in a special material which absorb higher energy light, make electrons jump from a ground level to an upper excitation level, thermally force electrons drop down to a lower excitation level and to fall electrons to the ground level with a delay time, is called “fluorescence”. The materials yielding fluorescence are called fluorescent material. Excited electrons return back to the ground state via a variety of excitation levels. Fluorescence has a wide spectrum containing a plurality of colors. Loss of energy which is a difference between the incidence light energy and the fluorescence energy is converted to heat.
Top electrodes 6 and 7 of the InGaN-LED 4 are joined to the lead 2 and a lead 10 by wires 8 and 9. Upper parts of the leads 2 and 10 and the fluorescent region 5 are encapsulated by a transparent resin 20. A dome-shaped white light emitting device is obtained. The InGaN-LED is built upon an insulating sapphire substrate which prohibits the LED from forming a cathode (n-electrode) on the bottom. Both cathode (n-electrode) and anode (p-electrode) are fabricated upon the top of the LED. Two top electrode requires two wires and a wide area per chip.
The GaN type white color device (A) obtains white light (W) by encapsulating a blue light InGaN-LED with a YAG-dispersed transparent resin 5, making blue light (B) by the InGaN LED, producing yellow fluorescence (Y) by the YAG excited by blue rays and synthesizing blue light with yellow light (W=B+Y). The YAG is doped with Ce. The InGaN-LED emits blue light of a 460 nm wavelength. The YAG fluoresces yellow light having a broad peak of a central wavelength of 570 nm. Namely the Ce-doped YAG converts 460 nm blue light into broad 570 nm peaked yellow light.
The on-sapphire InGaN LED has advantages of high luminosity and a long lifetime. The GaN-type white color device has also an advantage of a long lifetime. The YAG is an opaque fluorescent material. This is a weak point of (A), since absorption of blue light by the opaque YAG seriously attenuates blue light. Poor conversion efficiency of the YAG is another drawback. The white made by (A) is too weak. The faint white given by (A) is unsatisfactory. The GaN type white color devices (A) can produce weak white light of a color temperature of 7000K.
(B-type) ZnSe-type White Color Light Source Device (ZnCdSe-Emission, ZnSe Substrate (Fluorescent); FIG. 2)
Another white color semiconductor device (B) is a ZnSe-type device which had been proposed by the same applicant as,
{circle around (2)} Japanese Patent Application No.10-316169, “White Color LED”
The B type white color device includes neither an InGaN emission LED nor a YAG fluorescent material. FIG. 2 shows a section of the B-type white color emission device. The B-type employs a zinc selenide (ZnSe) substrate 22 instead of a sapphire substrate. An epitaxial active (emission) layer 23 of zinc cadmium selenide (ZnCdSe) and other ZnSe films are epitaxially grown on the ZnSe substrate 22. The ZnCdSe epitaxial emission layer produces blue light of a 485 nm wavelength by electron bandgap transition. Arrows B indicate the 485 nm blue light. The zinc selenide ZnSe substrate 22 is n-ZnSe doped with iodine (I), aluminum (Al), indium (In), gallium (Ga), chlorine (Cl) or bromine (Br). The inventors of {circle around (2)} had discovered the fact that the impurity I, Al, In, Ga, Cl or Br acts in ZnSe as a kind of fluorescence center. The inventors of {circle around (2)} had found that the dope impurity I, Al, In, Ga, Cl or Br absorbs the 485 nm blue light B yielded by the ZnCdSe epi-layer 23 and produces yellow light (arrows Y) with a broad spectrum of a 585 nm peak. The blue light B emitted from the ZnCdSe epi-layer and the yellow light Y provided from the ZnSe substrate 22 go upward. Human eyes sense white color light W emitting from the device by unifying the blue B and the yellow Y (W=B+Y).
In practice, a dome-shaped white color ZnCdSe LED device was produced by fitting the LED chip of FIG. 2 on a lead, wirebonding a top electrode to another lead and molding the leads and the chip with transparent resin. The B-type white color device made use of the substrate itself as a kind of fluorescent material. Namely the upper ZnCdSe epi-layer positively produces blue light and the bottom ZnSe substrate passively emits yellow light by converting blue light to yellow light. B-type needs no extra fluorescence material, because the substrate plays the role of a kind of fluorescent material.
A substrate is indispensable for a light emitting diode (LED) as a bench of physically supporting the light emission layers. The substrate of the white color ZnCdSe-LED has another role of a fluorescence material. The ZnCdSe-LED doubly makes the best use of the ZnSe substrate as a supporter and a fluorescent. Since the substrate plays the role of the fluorescence material, the ZnCdSe-LED dispenses with an independent fluorescence material like YAG. Omission of fluorescence material is an advantage of the ZnCdSe-LEDs.
The emission from the impurity-doped ZnSe is called “self-activated (SA) emission” which is a kind of fluorescence induced by the impurity I, Al, In, Ga, Cl or Br as emission centers. The white color ZnSe-LEDs succeeds in making white colors of arbitrary color temperature between 10000K and 2500K by making use of the 485 nm sharp blue light and the 585 nm peaked broad yellow SA light. Thinning the ZnSe substrate or lowering the dopant concentration in the ZnSe substrate produces cooler white of higher color temperature by reducing the yellow SA light. Thickening the ZnSe substrate or heightening the dopant concentration in the ZnSe substrate makes warmer white of lower color temperature by reinforcing the yellow SA light. A variety of white colors of arbitrary color temperatures can be obtained by changing a substrate thickness, dopant concentration or a Cd ratio in ZnCdSe.
There are three wide bandgap semiconductors ZnSe, SiC and GaN as a candidate of blue light LEDs as cited before. SiC had lost the race because of poor efficiency caused by indirect interband transition. ZnSe had been once prevailing, because bulk single crystals of ZnSe could be produced. But InGaN-LEDs on sapphire substrates is the single winner in the blue light race due to a long lifetime, high luminosity, low-cost and high energy (short wavelength) at present.
As aforementioned, blue light ZnSe-LEDs had lost the blue light LED race to InGaN because of a shorter lifetime and a longer wavelength (lower energy) than InGaN. However ZnSe-LEDs of an impurity doped fluorescent substrate have a rich probability of reviving as white light LEDs. The B-type ZnSe white color devices have advantages of low cost, small-size, because the ZnSe white color LEDs can eliminate a fluorescence material like YAG and a step of supplying an LED chip with an extra fluorescence material. One purpose of the present invention is to provide a white color light emitting device which excels in cost, color rendering properties, weight, size and lifetime.
The above-mentioned GaN-type white light device (A) (YAG/InGaN-LED) allows the InGaN-LED to make short wavelength blue light of 460 nm (point m in FIG. 3) and the Ce-doped YAG to fluoresce yellow light (point d in FIG. 3) with a 568 nm peak. Thus the GaN-type device (A) can synthesize any complex colors lying on straight line md in a chromaticity diagram in FIG. 3. The straight line md pierces the white color region encircled by a dotted curve. The YAG/InGaN-LED can synthesize white color light. The mentioned 7000K white means a point of X=0.31 and Y=0.32 within the white region on the chromaticity diagram. High color temperature derives from short wavelength blue of the InGaN-LED emission.
The other ZnSe-type white light source device (B) (ZnCdSe/ZnSe substrate) allows the ZnCdSe active layer to make long wavelength blue light of 485 nm (point j in FIG. 3) and the impurity (Al, In, Br, Cl, Ga or I) doped ZnSe to make yellow light with a 585 nm peak (point c in FIG. 3). Mixing of the 485 nm blue (j) with the 585 nm yellow (c) produces an arbitrary color lying on the line jc. Since the line jc traverses the white color region encircled by the dotted curve of FIG. 3, the ZnSe type (B) can yield a variety of white colors by changing the dopant concentration and the ZnSe substrate thickness.
The chromaticity diagram of FIG. 3 shows white colors of various color temperatures of 10000K, 8000K, 7000K, 6000K, 5000K, 4000K, 3000K and 2500K, which are all encircled by the dotted curve of the white region (W). A mild slanting of the line jc enables the ZnSe-type devices (B) to make a variety of white colors of different color temperatures. The ZnSe-type device (B) is superior to the InGaN-type device (A) in a rich variety of white colors.
[1. Advantages and Disadvantages of ZnSe-type White Color Light Source Devices (B)]
FIG. 3 shows the synthesis of white color in the ZnSe-type white color light source devices (B) by the line jc which connects the 495 nm point j (ZnCdSe-LED blue) to the 585 nm point c (ZnSe-substrate yellow Y). The line jc partly coincides with the curved locus of white color light from 10000K to 2500K in the white color region (W). The coincidence enables the ZnSe-type device to make a variety of white colors with different color temperatures from 10000K to 2500K by changing thickness of the ZnSe substrate or impurity concentration in the ZnSe substrate. This is a strong point of the ZnSe-type device. Another advantage is a simple layer structure, simple electrodes and a small size similar to an ordinary LED.
Definition of a main wavelength is described now by referring to the chromaticity diagram in FIG. 3. All actual color spots exist in a region encircled by a horseshoe-shaped main curve abcdefghijkmn and a pure-violet line npqa. Numerals affixed to the curve show wavelengths of dotted color spots on the curve. If an object color spot is on the horseshoe-shaped curve, the main wavelength of the object spot is the same wavelength of the spot itself. If an object color spot exists within the curve, the main wavelength is defined to be the wavelength of the point at which an extension of the line connecting the object color spot with white center (x=0.333 an y=0.333) crosses the horseshoe-curve. The before cited blue light (B) emitted from the ZnCdSe active layer has a main wavelength of 485 nm (point j). The yellow rays (Y) fluoresced from the ZnSe has a main wavelength of 585 nm (point c).
ZnSe-type blue light LEDs are suffering from rapid degeneration and short lifetime. High current density causes and increases defects in ZnSe-LEDs. Occurrence of many defects forces the ZnSe-LEDs to cease emitting light. Short lifetime is an inherent, unsolved weak point of ZnSe-LEDs.
The ratio of blue light (B) to yellow light (Y) is another significant problem for making white light (W) by mixing yellow with blue. When high energy 445 nm blue light (InGaN-LED) is employed as LED light, the ratio (B/Y) of necessary blue light takes the minimum value (nearly B:Y=1:1). Use of low energy 485 nm (point j) blue light (ZnCdSe-LED) forces the device to double the ratio of necessary blue light (nearly B:Y=2:1).
Blue light has less eye sensitivity than yellow light. ZnSe-type white light source device is inferior in emission efficiency, because ZnSe-type device requires much more blue light than GaN-LED type white light source devices.
[2. Advantages and Disadvantages of GaN-type White Color Light Source Devices]
On the contrary, GaN-type white color devices (InGaN-LED+YAG) have advantages of high energy blue light wavelength between 460 nm and 445 nm and a moderate ratio B:Y=1:1 which is about half blue light power of the ZnSe-type white color devices (B:Y=2:1). Besides, the GaN-type devices enjoy long lifetime.
However, the GaN-type white color devices are annoyed with disadvantages of heating-degeneration of a YAG fluorescence material and a transparent resin by the heat yielded in the InGaN-LED and in the YAG itself. Fluorescence materials yield heat which corresponds to the difference between the excitation energy and the fluorescence energy. The transparent resin enclosing the YAG has poor heat conductivity. The heat yielded from the LED and the YAG raises the temperature of the YAG and the resin. The heat degenerates the resin by inducing cracks, gaps or burns. Another problem of the GaN-type white color device is an improvement of the lifetime of the fluorescence material and the resin surrounding the device. A further problem is a low light output efficiency due to random scattering of light by particles of YAG.
[3. Significance of Color Rendering Property of White Color Light Source Devices]
What is important is a color rendering property when a white color light source is employed as illuminating light source. The color rendering property is a measure of estimating how much an object white color is akin to natural white. The color rendering property is a complex concept defined as 100% for an ideal incandescent lamp which has a broad spectrum covering blue, green, yellow, orange and red. Ordinary fluorescent tubes have about 80% color rendering properties. 80% is a threshold. White light should have a color rendering property higher than 80% in order to win the white color light source race over the ordinary fluorescence tubes.
Above-mentioned known white light sources (A) and (B), which convert a part of LED-emitted blue into yellow, are inferior in the color rendering property. Poor color rendering property prohibits the known devices (A) and (B) from acting as illuminating white light sources. Reasons why the known white light sources (A) and (B) have a bad color rendering property are described.    [Reason 1] Blue light emitting diodes (ZnSe-LEDs or InGaN-LEDs) emit monochromatic blue light with a narrow spectrum. The white color devices containing the blue light LEDs have a poor color rendering property.    [Reason 2] The yellow light converted from the blue light by the devices (A) and (B) lacks green and red components. The yellow fluoresced from the Ce-doped YAG does not include green and red components. The yellow made by the impurity-doped ZnSe also lacks a green component. If a new device replaces an incandescent bulbs as an illuminating light source, the new device should include a wide scope of spectrum having the red and green components.
One purpose of the present invention is to provide a white color light emitting device which prohibits generated heat from degenerating a transparent resin and fluorescence materials. Another purpose of the present invention is to provide a white color light emitting device which enjoys a long lifetime. A further purpose is to provide a white color light emitting device which can enhance the output efficiency by reducing random scattering by a fluorescent material. A further purpose of the present invention is to provide a white color light emitting device which gives natural white light superior in a color rendering property.
The present invention proposes a ultraviolet type white color light emitting device (Q) and a blue type white color light emitting device (R) which are sets of an inherent light emitting diode (LED) and one or two (ZnSSe, ZnS, ZnSe) fluorescent plates. The ultraviolet type white color device (Q) assembles an ultraviolet light emitting diode (LED) and two fluorescence plates which fluoresce blue light and yellow light. The blue light type white color device (R) contains a blue light emitting diode (LED) and a fluorescence plate which makes yellow fluorescence.
Ultraviolet type Q=ultraviolet LED+first fluorescence plate+second fluorescence plate.
Blue light type R=blue light LED+fluorescence plate.
Type Q makes use of double fluorescence phenomena. Type R depends upon a single fluorescence phenomenon. Type Q and type R contain photoactive parts which act within below-cited scopes of wavelengths.
    [Type Q] Ultraviolet LED=340 nm to 400 nm            First ZnS fluorescent plate=480 nm (peak wavelength)        Second ZnSxSe1-x fluorescent plate=585 nm (peak wavelength).            [Type R] Blue light LED=410 nm to 470 nm            ZnSxSe1-x fluorescent plate 568 nm to 580 nm        (heat-treated ZnSxSe1-x) x=0.3 to 0.67        (untreated ZnSxSe1-x) x=0.2 to 0.6[Q. Ultraviolet Type White Color Light Emitting Device (Ultraviolet LED+ZnS+ZnSe/ZnSSe)]        
An ultraviolet type white color light emitting device (Q) of the present invention contains an ultraviolet InGaN-LED, a first ZnS fluorescence plate, and a second ZnSe or ZnSSe fluorescence plate. Ultraviolet rays of the InGaN-LED excites the first ZnS fluorescence plate. The ZnS first fluorescence plate generates blue light. The blue light excites again the second ZnSe or ZnSSe fluorescence plate. The second ZnSe or ZnSSe fluorescence plate yields yellow light. The blue fluoresce from the first fluorescence plate and the yellow fluorescence from the second ZnSe/ZnSSe fluorescence plate mix together and make white color light with high color rendering property. ZnSSe is an abbreviation of ZnSxSe1-x (x: mixture rate).
Namely the white color light emitting device of the present invention consists of three emission elements.    A. Ultraviolet (UV) emitting InGaN-LED    B. Blue light (B) emitting first ZnS fluorescence plate    C. Yellow light (Y) emitting second ZnSSe(ZnSe) fluorescence plate    Output Light W Includes Only Blue Fluorescence and Yellow Fluorescence (W=B+Y).
The ultraviolet type (Q) employs ultraviolet ray light emitting diode (LED). Employment of invisible ultraviolet light LED for making white color features present invention. The ultraviolet rays should not emitted as output light, since invisible ultraviolet is of no use for synthesizing white. Whole of the ultraviolet rays produced by the InGaN-LED should be absorbed by the first ZnS fluorescence plate. All the ultraviolet power should be converted into blue fluorescence by the first ZnS fluorescence plate. Blue fluorescence excites the second ZnSe/ZnSSe fluorescence plate. The present invention makes the best use of fluorescence phenomena twice at two steps.
The gist of the ultraviolet type (Q) is the ultraviolet LED and two steps of fluorescence. No original ultraviolet rays, which is fully absorbed by the first fluorescence plates, are emitted outward. Two kinds of fluorescence (blue fluorescence and yellow fluorescence) emanate outward. In general, fluorescence has inherently a broad spectrum. Broad spectra favor the color rendering property which is a measure of estimating white color and is defined as 100% for natural incandescent lamps. The present invention proposes first an idea of synthesizing white light by combining two (blue and yellow) kinds of fluorescence. This is a quite novel invention.
The ultraviolet LED should emit ultraviolet rays of wavelengths between 340 nm and 400 nm. An InGaN-type LED having a high GaN rate can be an ultraviolet LED. ZnSe type LEDs having ZnCdSe active layers cannot make ultraviolet rays owing to narrow bandgaps.
Fluorescence has always a longer wavelength than that of the exciting light. A blue light LED is useless for making blue fluorescence. Production of blue fluorescence requires an independent light source capable of emitting light with higher energy or a lower wavelength. Fortunately, In1-yGayN-LEDs, which have been prevalently used as blue or green light LEDs, can be converted into ultraviolet LEDs by heightening a GaN rate y.
The gist of type (Q) is an ultraviolet LED and double fluorescence phenomena. Ultraviolet rays are all absorbed in the fluorescence plates. Two kinds of fluorescence (blue and yellow) are emitted outward from type (Q). Fluorescence has inherently a wide spectrum which is an advantage for a color rendering property. The white containing wide spectra having broad yellow and blue components is superior in the color rendering property.
[R. Blue Type White Color Light Emitting Device (Blue Light LED+Fluorescent Plate)]
R1. The present invention proposes a blue type white color light emitting device (R) having an InGaN-LED and a bulk/powder ZnSSe fluorescence plate piled upon the InGaN-LED. The InGaN-LED emits blue light (B). The ZnSSe fluorescence plate, which is either a single crystal or polycrystal bulk or powder solidified plate by a water-resistant transparent resin, absorbs blue light rays and produces yellow fluorescence (Y). Namely the ZnSSe fluorescence plate converts a part of blue light into yellow light with a broad spectrum. The ZnSSe/InGaN light source of the present invention makes white color light by mixing the yellow fluorescence (Y) with the blue light (B) (W=B+Y).
The InGaN-LED can be replaced by another blue light LED. This invention proposes another white color light source having a blue light LED other than InGaN-LED and a bulk/powder ZnSSe fluorescence plate piled upon the blue light LED.
R2. 410 nm-470 nm blue light emitted from a blue light LED
Blue light between 410 nm and 470 nm is high energy blue with short wavelengths. The 410 nm-470 nm blue corresponds to a lowest part nm of a blue region of a chromaticity diagram in FIG. 3. Such a short wavelength blue light cannot be produced by ZnSe LED having ZnCdSe active layer which emits 485 nm blue (point j). InGaN-LEDs are preferable candidates for the blue light making LED, since the InGaN-LEDs can produce short wavelength blue light of 410 nm to 470 nm. The weight of blue light in synthesized white can be controlled by changing a driving current of the InGaN LED. On-sapphire InGaN LEDs excel in lifetime, cost, reliability and utility. The mentioned known reference (A) used resin-diffused YAG which had been well known as a fluorescence material. But this invention does not employ the YAG unlike the known device (A). The present invention uses another material ZnSSe which has never known as fluorescent material before this invention.
R3. 568 nm-580 nm yellow fluoresced by ZnSSe
The 410 nm-470 nm blue light and 568 nm-580 nm yellow make white color light of an arbitrary color temperature between 3000K and 10000K.
R4. Impurity-doped ZnSxSe1-x as fluorescence Material
ZnSxSe1-x is a mixture of ZnS and ZnSe. A suitable range of a ZnS ratio x will be give later. The ratio x is often omitted for simplicity in this description. Pure ZnSSe does not fluoresce. ZnSSe obtains fluorescence performance by doping some impurity which becomes a emission center in ZnSSe. Suitable impurities are aluminum(Al), indium(In), gallium(Ga), chlorine(Cl), bromine(Br), iodine(I). The ZnSSe plate as a fluorescence material employed by the present invention should include at least one of Al, In, Ga, Br, Cl or I at a concentration higher than 1×1017 cm−3. Doping of the impurity less than 1×1017 cm−3 cannot cause sufficient fluorescence. The weight of yellow light in synthesized white light can be varied by changing the impurity concentration and the thickness of the fluorescence plate.
The aforementioned known reference (B) has employed an impurity doped ZnSe substrate as a fluorescence material. Instead of ZnSe, the present invention uses ZnSSe, a mixture of ZnS and ZnSe, as a fluorescence material. Nobody has known that impurity-doped ZnSSe acts as fluorescent material before the present invention.
Instead of 485 nm of a ZnCdSe-LED of {circle around (2)}, the present invention employs an exciting light source (e.g., InGaN-LED) of 410 nm to 470 nm, which are shorter than 485 nm. Other 410-470 nm light sources else than InGaN-LED can be available. The present invention makes use of a ZnSSe bulk plate or a power-solidified plate. The ZnSSe bulk means a single crystal bulk or a polycrystal bulk. The power-solidified plate means a plate constructed by a transparent resin dispersed with ZnSSe powder. ZnS and ZnSe have an inherent drawback of weak water resistance (water-absorptive). A single crystal ZnSSe is the best, since single crystal ZnSSe has the highest water-resistance, the lowest scattering the highest heat conduction and the least degeneracy. A polycrystal ZnSSe is the next best. A polycrystal having greater grains is better than another polycrystal having smaller grains. Bigger grains enable the polycrystal ZnSSe to reduce light scattering, water absorption, degeneration and to heighten heat conductivity and lifetime. Powder-solidified ZnSSe, which is dispersed into a transparent resin or glass, has disadvantages of poor water resistance, random light scattering, degeneration, low heat diffusion and short lifetime. High heat conduction of the bulk ZnSSe enables the fluorescence plate to release the heat induced into the ZnSSe plate quickly than other resin materials like epoxi resin or Si-resin. This works of the ZnSSe plate contribute to control heating and degeneracy.
ZnSSe (single or poly-crystals) bulks have another merit of good controllability of refraction and reflection of light at surfaces and high efficiency of outputting blue and yellow rays. However, ZnSSe bulks (single or poly-crystals) have drawbacks of difficult production and high cost.
Powder-solidified ZnSSe is suffering from low efficiency and short lifetime. However, powder-solidified ZnSSe has advantages of low cost and facile production. ZnSSe powder can be dispersed into an outer transparent resin for molding instead of an independent plate. In this case, processes of molding by resin and making the plate are done at the same time.
R5. ZnSe has a narrower bandgap. ZnS has a wider bandgap. An intermediate material having an intermediate bandgap between ZnSe and ZnS can be made by changing the rate x of ZnS and the rate 1-x of ZnSe. A higher x realizes a higher bandgap, which induces yellow of a shorter wavelength. In the case of heat-treated ZnSSe fluorescence plates, a suitable range of x is from 0.3 to 0.67 (0.3≦x≦0.67). In the case of untreated ZnSSe fluorescence plates, a suitable range of x is 0.2 to 0.6 (0.2≦x≦0.6).
R6. Bulk ZnSSe is preferable for fluorescence plates. Furthermore, it is desirable that an average grain size of a ZnSSe polycrystal is larger than the thickness of the plate.
A polycrystal containing small grains have many grain boundaries, which have functions of leading water and scattering light causing optical loss. Large grains prevent water from infiltrating into the fluorescence plate. Large grains reduce random scattering of light at grain boundaries. The polycrystal having grains of sizes wider than thickness of the plate are suitable. Preferably, all grains are single in the direction of thickness.
R7. The best choice is a ZnSSe single crystal fluorescence plate. Single crystal is free from grains and grain boundaries which induce scattering, water infiltrating and degeneration. However, it is difficult to make single crystal of ZnSSe. Only a chemical vapor transportation method (CVT) is a practical method for making single crystal ZnSSe at present. But it takes long time to grow single crystal by CVT. Instead of high cost single crystal, bulk polycrystals are next favorable. It is not easy to make good polycrystalline ZnSSe. ZnSSe polycrystals, which are not low-cost yet, can be made by chemical vapor deposition method (CVD). Low cost fluorescence plates can be obtained by solidifying ZnSSe powder with a transparent resin or a glass.
R8. 410 nm-470 nm blue light is required. Some InGaN-LEDs can produce blue light of the range. Other LEDs than InGaN-LEDs can be utilized for the blue light source between 410 nm and 470 nm.
R9. Blue light wavelength λLED emitted from the LED should satisfy an inequality of λLED≦1239/(2.65+1.63x−0.63x2), where x is the rate of ZnS and (1-x) is the rate of ZnSe in the fluorescence material. ZnSe (x=1) has a bandgap of 2.7 eV and an absorption edge wavelength of 460 nm. ZnS has a bandgap of 3.7 eV and an absorption edge wavelength of 335 nm. A bandgap energy of a mixture ZnSxSe1-x is give by Eg=2.7+1.63x−0.63x2, which is different from the denominator by 0.05. An absorption edge wavelength λg is calculated by dividing 1239 (=hc) by a bandgap Eg. The denominator in the inequality is different at the constant term of 2.65 from Eg (2.7). The above inequality of λLED requires that the ZnSSe fluorescent plate should be excited by the blue light having lower energy (longer wavelength) than the bandgap of the ZnSSe. If ZnSSe were excited by light higher than the bandgap, ZnSSe itself emits bandgap transition light (blue) instead of the doped impurity, which could not make white light.
Inequality λLED signifies that the blue light emitted from the InGaN-LED can reach inner portions of the ZnSSe fluorescence plate with least attenuation caused by the bandgap emission. In general, a semiconductor absorbs light whose energy is bigger than the bandgap and emits light of bandgap wavelength. If the blue light from the LED has energy larger than the bandgap, the blue light are absorbed and converted into blue light of the bandgap, which is a loss for the purpose of making yellow. Inequality λLED forbids the bandgap emission in ZnSSe.
R10. Non-treated ZnSSe is available. However, heat-treatment in Zn atmosphere is effective for ZnSSe for reducing scattering or non-fluorescent absorption. Peak wavelengths and intensities are varied by the heat-treatment. An available x ranges from 0.3 to 0.67 (x=0.3-0.67) for heat-treated ZnSxSe1-x. A suitable range of x is between 0.2 and 0.6 (x=0.2-0.6) for untreated ZnSxSe1-x.